A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that example, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution:
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            
where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from the Rayleigh criterion that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength, and thus reduce the critical dimension, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are configured to output a radiation wavelength of about 13 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Possible sources of EUV radiation include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. When using a plasma source, contamination particles are created as a by-product of the plasma generation. Generally, such contamination particles are undesired because they adhere for example to reflective surfaces of the lithographic apparatus. Build up of contamination particles on reflective surfaces of the lithographic apparatus reduces the reflectivity of those surfaces, and consequently reduces the achievable throughput of the lithographic apparatus.
It is desirable to reduce the accumulation of contamination particles on reflective surfaces of the lithographic apparatus.